How far into fantasy do you want to go with this?  Can these pirates work with their sea god to hide fully manned ships underwater?  Does the weather or the tides always favor the pirates?

If we go for more verisimilitude here are some questions to consider.

How much of the islands economy does pirating make up.  It is clear that the island lords profit from piracy, but how does the food get on the forks?  Do they farm on the island? Is all the food imported or captured?

How do they move their pirated goods? Are they in bed with various merchants or do the pirate's lord loyalities favor certain nations or ethnicities?  Say do they mostly steal from the Spanish and primarily sell to the Dutch.

How long has this island been around?

What is the islands history?

There are lot of things you could play up, but from what you wrote I feel like you want to focus on the sea god relationship. Maybe should focus your write up on the sea god and his followers.  Then you can just mention that they indulge in piracy.

Who?

terminate

mussolini

Fibonacci